1. Field of the Invention
This invention relates generally to audio enhancement systems, and especially those systems and methods designed to improve the realism of stereo sound reproduction. More particularly, this invention relates to apparatus for overcoming the acoustic deficiencies of a sound system as perceived by a listener which can result when speakers within the sound system are not ideally positioned.
2. Description of the Related Art
In a sound reproduction environment various factors may serve to degrade the quality of reproduced sound as perceived by a listener. Such factors distinguish the sound reproduction from that of an original sound stage. One such factor is the location of speakers in a sound stage which, if inappropriately placed, may lead to a distorted sound-pressure response over the audible frequency spectrum. The placement of speakers also affects the perceived width of a soundstage. For example, speakers act as point sources of sound limiting their ability to reproduce reverberant sounds that are easily perceived in a live sound stage. In fact, the perceived sound stage width of many audio reproduction systems is limited to the distance separating a pair of speakers when placed in front of a listener. Another factor degrading the quality of reproduced sound may result from microphones which record sound differently from the way the human hearing system perceives sound. In an attempt to overcome the factors which degrade the quality of reproduced sound, countless efforts have been expended to alter the characteristics of a sound reproduction environment to mimic that heard by a listener in a live sound stage.
Some efforts at stereo image enhancement have focused on the acoustic abilities and limitations of the human ear. The human ear's auditory response is sensitive to sound intensity, phase differences between certain sounds, the frequency of the sound itself, and the direction from which sound emanates. Despite the complexity of the human auditory system, the frequency response of the human ear is relatively constant from person to person.
When sound waves having a constant sound pressure level across all frequencies are directed at a listener from a single location, the human ear will react differently to the individual frequency components of the sound. For example, when sound of equal sound pressure is directed towards a listener from in front of the listener, the pressure level created within the listener's ear by a sound of 1000 hertz will be different from that of 2000 hertz.
In addition to frequency sensitivity, the human auditory system reacts differently to sounds impinging upon the ear from various angles. Specifically, the sound pressure level within the human ear will vary with the direction of sound. The shape of the outer ear, or pinna, and the inner ear canal are largely responsible for the frequency contouring of sounds as a function of direction.
The human auditory response is sensitive to both azimuth and elevation changes of a sound's origin. This is particularly true for complex sound signals, i.e., those having multiple frequency components, and for higher frequency components in general. The variance in sound pressure within the ear is interpreted by the brain to provide indications of a sound's origin. When a recorded sound is reproduced, the directional cues to the sound's origin, as interpreted by the ear from sound pressure information, will thus be dependent upon the actual location of speakers that reproduce the sound.
A constant sound pressure level, i.e., a “flat” sound pressure versus frequency response, can be obtained at the ears of a listener from loudspeakers positioned directly in front of the listener. Such a response is often desirable to achieve a realistic sound image. However, the quality of a set of speakers may be less than ideal, and they may not be placed in the most acoustically-desirable location. Both such factors often lead to disrupted sound pressure characteristics. Sound systems of the prior art have disclosed methods to “correct” the sound pressure emanating from speakers to create a spatially correct response thereby improving the resulting sound image.
To achieve a more spatially correct response for a given sound system, it is known to select and apply head-related-transfer-functions (HRTFs) to an audio signal. HRTFs are based on the acoustics of the human hearing system. Application of an HRTF is used to adjust the amplitudes of portions of the audio signal to compensate for spatial distortion. HRTF-based principles may also be used to relocate a stereo image from non-optimally placed loudspeakers.
The efforts made in the prior art to correct acoustic deficiencies within an audio reproduction system have often focused on the deficiencies present in automobile sound systems. One such attempt is disclosed in both U.S. Pat. No. 4,648,117 issued to Kunugi, et al., and U.S. Pat. No. 4,622,691 issued to Tokumo, et al. In the disclosures of Kunugi and Tokumo, a system for correcting sound absorption levels and for avoiding sound wave interference is described for use within a vehicle. The disclosed system includes a sound-pressure correcting circuit and a signal-delay circuit for achieving the desired frequency response. The sound-pressure correction is achieved by a high-frequency boost of the sound signal applied in three stages. The first stage is a high-frequency correction for the average sound absorption factor of a vehicle, the second high-frequency correction stage is dependent upon the sound absorption factor of a specific vehicle, and the third high-frequency correction factor is dependent upon the number of passengers seated within the vehicle.
In U.S. Pat. No. 5,146,507 issued to Satoh et al., an audio reproduction system control device is disclosed for correcting the frequency response of a given reproduction environment to match that of a standard frequency response characteristic. The system in Satoh provides a correction parameter for sound signals directed to front left, front right, rear left and rear right speakers of a sound field, such as in an automobile. Prestored acoustic characteristics relating to frequency and reflection are utilized to adapt the audio reproduction control device to a variety of sound environments.
Another system designed to modify a frequency response characteristic within an automobile is disclosed in U.S. Pat. No. 4,888,809 issued to Knibbeler. The system of Knibbeler attempts to create a flat frequency response at two separate non-coincident listening positions, such as the front and rear positions in an automobile passenger compartment, by adjusting a pair of filter units. Each of the filter units receives an input signal and affects an output signal delivered to a corresponding sound transducer.
Still other patents disclose sound systems, which alter an audio signal to equalize the frequency response. Such patents include U.S. Pat. No. 5,371,799 issued to Lowe, et al., U.S. Pat. No. 5,325,435 issued to Date, et al., U.S. Pat. No. 5,228,085 issued to Aylward, U.S. Pat. No. 5,033,092 issued to Sadaie, U.S. Pat. No. 4,393,270 issued to van den Berg, and U.S. Pat. No. 4,329,544 issued to Yamada.